LeetCode 117. Populating Next Right Pointers in Each Node II

117. Populating Next Right Pointers in Each Node II

Difficulty: Medium

Given a binary tree

struct Node {
  int val;
  Node *left;
  Node *right;
  Node *next;
}

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Example:

Input: {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":null,"right":null,"val":4},"next":null,"right":{"$id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"$id":"5","left":null,"next":null,"right":{"$id":"6","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}

Output: {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":{"$id":"4","left":null,"next":{"$id":"5","left":null,"next":null,"right":null,"val":7},"right":null,"val":5},"right":null,"val":4},"next":{"$id":"6","left":null,"next":null,"right":{"$ref":"5"},"val":3},"right":{"$ref":"4"},"val":2},"next":null,"right":{"$ref":"6"},"val":1}

Explanation: Given the above binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B.

Note:

• You may only use constant extra space.
• Recursive approach is fine, implicit stack space does not count as extra space for this problem.

Solution

Language: Java

/*
// Definition for a Node.
class Node {
      public int val;
      public Node left;
      public Node right;
      public Node next;
​
      public Node() {}
​
      public Node(int _val,Node _left,Node _right,Node _next) {
            val = _val;
            left = _left;
            right = _right;
            next = _next;
      }
};
*/
class Solution {
      public Node connect(Node root) {
            if (root == null) {
                  return root;
           }
            Node nextRoot = connectLine(root);
            connect(nextRoot);
            return root;
     }
      
      private Node connectLine(Node root) {
            if (root == null) {
                  return null;
           }
            Node cur = root;
            Node dummy = new Node();
            Node nextCur = dummy;
            while (cur != null) {
                  if (cur.left != null) {
                        nextCur.next = cur.left;
                        nextCur = nextCur.next;
                 }
                  if (cur.right != null) {
                        nextCur.next = cur.right;
                        nextCur = nextCur.next;
                 }
                  cur = cur.next;
           }
            return dummy.next;
            
     }
}